What is the purpose of flutter testing?

Question

All I am aware of is that flutter testing involves the aircraft shaking violently. How is a flutter test performed and what is its purpose?

Answer 1

Flutter is a phenomenon that can occur when a structure is subjected to aerodynamic forces. It occurs not only in aircraft but also for example in buildings, power lines, road signs and bridges. Flutter is an oscillation caused by interaction of aerodynamic forces, structural elasticity and inertial effects.

Perhaps the best known example of flutter is the bridge over the Tacoma Narrows ('Gallopin' Gertie') that collapsed due to flutter in gale force winds.

Flutter in aircraft causes the wings and / or stabilizers to oscillate. When the airspeed increases the energy added in each oscillation to the structure by the aerodynamic forces increases. At some speed the damping of the structure may be insufficient to absorb the energy increase from the aerodynamic loads and the amplitude of the harmonic oscillations will grow until the structure breaks.

A flutter test is performed to determine what speed flutter occurs yet is still damped. Theoretically an aircraft should be able to stay structurally intact up to the design dive speed ($V_d$) which is at least 1.4 times the design cruise speed. In flight tests, the $V_d$ is not always demonstrated as this is very risky since there would not be any margin from breaking up. The maximum speed is slowly increased in subsequent flight tests with careful data analyses in between up to a point where the test team decides that it is no longer safe to continue. This establishes the maximum demonstrated dive speed ($V_{df}$); the published maximum operating speed ($V_{mo}$) and never-exceed speed ($V_{ne}$) are well below $V_{df}$ to give margin from disaster.

This video shows flutter in a Hawker 800. The incident occurred not during testing but was caused by inadequate fitting of the ailerons^{(FAA PDF)}.

Answer 2

Aeroelastic flutter is a dynamic instability of an elastic structure where there is feeding between the deformation of the structure and the force provided by the aerodynamics.

Is a very dangerous condition where the vibration on the structure keeps growing and so the stress over it, endangering safe flight. It is a condition that needs to be completely avoided in a commercial flight.

Airplane manufacturers need to provide information to airlines of the operation envelope of the airplane, and part of that information is to include region of flutter to be avoided (or maximum time on it).

For providing such information manufacturers predict using internal methods (usually simulation) the flutter speed for each condition, but certification authorities request that a physical test is perform to detect the flutter velocity which will be the final speed provided in the airplane manuals.

How is made? Taking an specific airplane weight the flight test pilots approach steadily step by step to the flutter speed until certain level of vibration is obtained, determining the final airplane flutter speed.